It has been recently documented that one of the significant factors in the child's mastery of false belief, is their understanding and use of mental- state language, specifically complementation. These studies, based on spoken English, argue that the syntactic form of complementation provides the necessary underpinnings for false-belief understanding. If, however, the syntactic form of complementation is articulated differently or not at all in a language, performance on false-belief tasks could vary from what has been previously reported. My project proposal seeks to examine adult productions of mental language in American Sign Langauge (ASL) in order to investigate alternative ways language about the mind can be conveyed. This proposal will further explore this issue in a newly emerging language, Nicaraguan Sign Language (NSL), where complementation has yet to emerge. These studies combined seek to prove that it is not the syntactic form, but rather the conceptual underpinnings of complementation that facilitate the acquisition of false-belief. From the native-signing American adults, I will elicit a series of narratives designed to target mental language. Early results demonstrate use of structures that are syntactically different but conceptually similar to complementation. Secondly, to gain an understanding of what linguistic structures emerged in a newly evolving language which could play a role in the acquisition of false belief, I will elicit the same narratives from the earliest Nicaraguan signers, correlating the their language with their performance on a non-verbal false-belief task. Developmental evidence will come from the administration of the same tasks in a group of young signers of NSL. Combined these studies will enhance our understanding of the development and use of mental language in signed languages and its potential relationship to cognitive development, specifically the understanding of false-belief.